Adeno-associated viruses (AAV) have unique features that make them attractive as vectors for gene therapy. Adeno-associated viruses infect a wide range of cell types. However, they are non-transforming, and are not implicated in the etiology of any human disease. Introduction of DNA to recipient host cells generally leads to long-term persistence and expression of the DNA without disturbing the normal metabolism of the cell.
There are at least three desirable features of a recombinant AAV vector preparation for use in gene transfer, especially in human gene therapy. First, it is preferred that the vector should be generated at titers sufficiently high to transduce an effective proportion of cells in the target tissue. Gene therapy in vivo typically requires a high number of vector particles. For example, some treatments may require in excess of 108 particles, and treatment of cystic fibrosis by direct delivery to the airway may require in excess of 1010 particles. Second, it is preferred that the vector preparations should be essentially free of replication-competent AAV (i.e. phenotypically wild-type AAV which can be replicated in the presence of helper virus or helper virus functions). Third, it is preferred that the rAAV vector preparation as a whole be essentially free of other viruses (such as a helper virus used in AAV production) as well as helper virus and cellular proteins, and other components such as lipids and carbohydrates, so as to minimize or eliminate any risk of generating an immune response in the context of gene therapy. This latter point is especially significant in the context of AAV because AAV is a “helper-dependent” virus that requires co-infection with a helper virus (typically adenovirus) or other provision of helper virus functions in order to be effectively replicated and packaged during the process of AAV production; and, moreover, adenovirus has been observed to generate a host immune response in the context of gene therapy applications (see, e.g., Byrnes et al., Neuroscience 66:1015, 1995; McCoy et al., Human Gene Therapy 6:1553, 1995; and Barr et al., Gene Therapy 2:151, 1995). The methods of the present invention address these and other desirable features of rAAV vector preparations, as described and illustrated in detail below.
General reviews of AAV virology and genetics are available elsewhere. The reader may refer inter alia to Carter, “Handbook of Parvoviruses”, Vol. I, pp. 169-228 (1989), and Berns, “Virology”, pp. 1743-1764, Raven Press, (1990). AAV is a replication-defective virus, which means that it relies on a helper virus in order to complete its replication and packaging cycle in a host cell. Helper viruses capable of supporting AAV replication are exemplified by adenovirus, but include other viruses such as herpes and pox viruses. The AAV genome generally comprises the packaging genes rep and cap, with other necessary functions being provided in trans from the helper virus and the host cell.
AAV particles are comprised of a proteinaceous capsid having three capsid proteins, VP1, VP2 and VP3, which enclose a ˜4.6 kb linear single-stranded DNA genome. Individual particles package only one DNA molecule strand, but this may be either the plus or minus strand. Particles containing either strand are infectious, and replication occurs by conversion of the parental infecting single strand to a duplex form, and subsequent amplification, from which progeny single strands are displaced and packaged into capsids. Duplex or single-strand copies of AAV genomes (sometimes referred to as “proviral DNA” or “provirus”) can be inserted into bacterial plasmids or phagemids, and transfected into adenovirus-infected cells.
By way of illustration, the linear genome of serotype AAV2 is terminated at either end by an inverted terminal repeat (ITR) sequence. Between the ITRs are three transcription promoters p5, p19, and p40 that are used to express the rep and cap genes (Laughlin et al., 1979, Proc. Nati. Acad. Sci. USA, 76:5567-5571). ITR sequences are required in cis and are sufficient to provide a functional origin of replication, integration into the cell genome, and efficient excision and rescue from host cell chromosomes or recombinant plasmids. The rep and cap gene products provide functions for replication and encapsidation of viral genome, respectively, and it is sufficient for them to be present in trans.
The rep gene is expressed from two promoters, p5 and p19, and produces four proteins designated Rep78, Rep68, Rep52 and Rep40. Only Rep78 and Rep68 are required for AAV duplex DNA replication, but Rep52 and Rep40 appear to be needed for progeny, single-strand DNA accumulation (Chejanovsky et al., Virology 173:120, 1989). Rep68 and Rep78 bind specifically to the hairpin conformation of the AAV ITR and possess several enzyme activities required for resolving replication at the AAV termini. Rep78 and Rep68, also exhibit pleiotropic regulatory activities including positive and negative regulation of AAV genes and expression from some heterologous promoters, as well as inhibitory effects on cell growth. The cap gene encodes capsid proteins VP1, VP2, and VP3. These proteins share a common overlapping sequence, but VP1 and VP2 contain additional amino terminal sequences transcribed from the p40 promoter by use of alternate initiation codons. All three proteins are required for effective capsid production.
AAV genomes have been introduced into bacterial plasmids by procedures such as GC tailing (Samulski et al., 1982, Proc. Natl. Acad. Sci. USA, 79:2077-2081), addition of synthetic linkers containing restriction endonuclease cleavage sites (Laughlin et al., 1983, Gene, 23:65-73) or by direct, blunt-end ligation (Senapathy & Carter, 1984, J. Biol. Chem., 259:4661-4666). Transfection of such AAV recombinant plasmids into mammalian cells with an appropriate helper virus results in rescue and excision of the AAV genome free of any plasmid sequence, replication of the rescued genome and generation of progeny infectious AAV particles.
Recombinant AAV vectors comprising a heterologous polynucleotide of therapeutic interest may be constructed by substituting portions of the AAV coding sequence in bacterial plasmids with the heterologous polynucleotide. General principles of rAAV vector construction are also reviewed elsewhere. See, e.g., Carter, 1992, Current Opinions in Biotechnology, 3:533-539; and Muryczka, 1992, Curr. Topics in Microbiol. and Immunol., 158:97-129). The AAV ITRs are generally retained, since packaging of the vector requires that they be present in cis. However, other elements of the AAV genome, in particular, one or more of the packaging genes, may be omitted. The vector plasmid can be packaged into an AAV particle by supplying the omitted packaging genes in trans via an alternative source.
In one approach, the sequence flanked by AAV ITRs (the rAAV vector sequence), and the AAV packaging genes to be provided in trans, are introduced into the host cell in separate bacterial plasmids. Examples of this approach are described in Ratschin et al., Mol. Cell. Biol. 4:2072 (1984); Hermonat et al., Proc. Natl. Acad. Sci. USA, 81:6466 (1984); Tratschin et al., Mol. Cell. Biol. 5:3251 (1985); McLaughlin et al., J. Virol., 62:1963 (1988); and Lebkowski et al., 1988 Mol. Cell. Biol., 7:349 (1988). Samulski et al. (1989, J. Virol., 63:3822-3828) have described a packaging plasmid called pAAV/Ad, which consists of Rep and Cap encoding regions enclosed by ITRs from adenovirus. Human airway epithelial cells from a cystic fibrosis patient have been transduced with an AAV vector prepared using the pAAV/Ad packaging plasmid and a plasmid comprising the selective marker gene neo expressed via the AAV p5 promoter (Flotte et al., Am. J. Respir. Cell. Mol. Biol. 7:349, 1992).
A second approach is to provide either the vector sequence, or the AAV packaging genes, in the form of an episomal plasmid in a mammalian cell used for AAV replication. For example, U.S. Pat. No. 5,173,414 describes a cell line in which the vector sequence is present as a high-copy episomal plasmid. The cell lines can be transduced with the trans-complementing AAV functions rep and cap to generate preparations of AAV vector. This approach is not ideal, because the copy number per cell cannot be rigorously controlled and episomal DNA is much more likely to undergo rearrangement, leading to production of vector byproducts.
A third approach is to provide either the vector sequence, or the AAV packaging genes, or both, stably integrated into the genome of the mammalian cell used for replication.
One exemplary technique is outlined in international patent application WO 95/13365 (Targeted Genetics Corporation and Johns Hopkins University) and corresponding U.S. Pat. No. 5,658,776 (by Flotte et al.). This example uses a mammalian cell with at least one intact copy of a stably integrated rAAV vector, wherein the vector comprises an AAV ITR and a transcription promoter operably linked to a target polynucleotide, but wherein the expression of rep is limiting. In a preferred embodiment, an AAV packaging plasmid comprising the rep gene operably linked to a heterologous AAV is introduced into the cell, and then the cell is incubated under conditions that allow replication and packaging of the AAV vector sequence into particles.
A second exemplary technique is outlined in patent application WO 95/13392 (Trempe et al.). This example uses a stable mammalian cell line with an AAV rep gene operably linked to a heterologous promoter so as to be capable of expressing functional Rep protein. In various preferred embodiments, the AAV cap gene can be provided stably as well or can be introduced transiently (e.g. on a plasmid). A recombinant AAV vector can also be introduced stably or transiently.
Another exemplary technique is outlined in patent application WO 96/17947 (by Targeted Genetics Corporation, J. Allen). This example uses a mammalian cell which comprises a stably integrated AAV cap gene, and a stably integrated AAV rep gene operably linked to a heterologous promoter and inducible by helper virus. In various preferred embodiments, a plasmid comprising the vector sequence is also introduced into the cells (either stably or transiently). The rescue of AAV vector particles is then initiated by introduction of the helper virus.
Other methods for generating high-titer preparations of recombinant AAV vectors have been described. International Patent Application No. PCT/US98/18600 describes culturing a cell line which can produce rAAV vector upon infection with a helper virus; infecting the cells with a helper virus, such as adenovirus; and lysing the cells. AAV and other viral production methods and systems are also described in, for example, WO 97/09441 (PCT/US96/14423); WO 97/08298 (PCT/US96/13872); WO 97/21825 (PCT/US96/20777); WO 97/06243 (PCT/FR96/01064); WO 99/11764; Perrin et al. (1995) Vaccine 13:1244-1250; Paul et al. (1993) Human Gene Therapy 4:609-615; Clark et al. (1996) Gene Therapy 3:1124-1132.
These various examples address the issue of providing AAV at sufficiently high titer, minimizing recombination between vector and packaging components, and reducing or avoiding the potential difficulties associated with the expression of the AAV rep gene in mammalian cell line (since the Rep proteins can not only limit their own expression but can also affect cellular metabolism). However, packaging of an AAV vector into viral particles still relies on the presence of a suitable helper virus for AAV or the provision of helper virus functions. Helper viruses capable of supporting AAV replication are exemplified by adenovirus, but include other viruses such as herpes and pox viruses. The presence of significant quantities of infectious helper virus in a preparation of AAV vectors is problematic in that the preparation is intended for use in human administration. Even the presence of non-replicative helper virus components can cause an unacceptable immunological reaction in the treated subject.
The potential problems elicited by helper virus antigen have been illustrated in several recent studies. Byrnes et al. (Neuroscience 66:1015, 1995) injected an E1-region deleted, non-replicating human adenovirus type 5 into the brains of inbred rats. An inflammatory response was observed that was attributed to the particles administered rather than to expression of new viral proteins due to viral replication in the cells. Presence of the virus was associated with an increase in MHC Class I gene expression and a heavy infiltration of macrophages and T cells. McCoy et al. (Human Gene Therapy 6:1553, 1995) instilled the lungs of mice with intact adenovirus, adenovirus with incomplete genomes, or adenovirus inactivated with ultraviolet light. All induced pulmonary inflammation, and the number of inflammatory cells in the lung tissue was quantitatively similar for all three forms of the virus. Comparative experiments using adenovirus constructs in normal and immune-deficient mice performed by Barr et al. (Gene Therapy 2:151, 1995) indicate that the anti-adenovirus immune response is primarily T-cell mediated and gives rise to a memory response that affects subsequent doses.
Accordingly, in the development of recombinant AAV vectors such as those for use in gene therapy, there is a need for strategies that minimize the amount of helper virus, as well as helper virus proteins and cellular proteins, present in the final preparation, while at the same time still achieving a high titer of AAV so that the methods can be effectively employed on a scale that is suitable for the practical application of gene therapy techniques.
Since high titers of rAAV vector preparations are particularly useful, but the production of high titers of rAAV, particularly in large-scale procedures, can lead to the generation of significant quantities of contaminating helper virus (e.g. adenovirus or “Ad”), helper virus proteins (e.g. Ad proteins), and/or cellular proteins, it became especially important to design scalable methods for the production of rAAV that can be used for the generation of high-titer preparations that are substantially free of contaminating virus and/or viral or cellular proteins.
Prior art methods used to produce recombinant AAV particle using packaging cells required a cell lysis step due to the pervasive belief that AAV is not released from producer cells in any appreciable amount without lysing the cells. See, for example, Chirico and Trempe (1998) J. Viral. Methods 76:31-41. However, the cell lysate contains various cellular components which must be separated from the rAAV vector before it is suitable for in vivo use.
The present disclosure provides methods for achieving high-titer production of rAAV vectors, including rAAV released from a producer cell without lysing the cell(s), and demonstrates that such techniques can be employed for the large-scale production of recombinant AAV vector preparations.